1. Field of the Invention
The present invention relates to image reading apparatus which read images optically and change the read image into an electrical signal, and relates especially to image reading apparatus using image sensors almost all of whose pixels are read out, and the output of only a few of whose pixels is discarded.
2. Related Art
A conventional image reading apparatus usually uses a CCD (Charge Coupled Device) sensor. The CCD sensor is designed so that it has a larger number of pixels than are used (i.e., the output of some pixels is discarded; this is sometimes referred to as the sensor imaging size being larger than the actual image size to be read out). To throw that signal away the apparatus controls the electrical timing of reading out the signal of the CCD sensor. It will be appreciated that if the sensor image size is much larger than the actual imaging size, the image sensor is being used wastefully.
On the other hand, if the sensor image size is only slightly larger than the actual imaging size, there are the following problems:
(1) In order to manufacture the image reading apparatus without adjusting the reading out timing of the sensor, the apparatus, especially the optical system used with the sensor, must be assembled with high precision. This raises the cost.
(2) If the image reading apparatus is assembled without high precision, the reading out timing of the apparatus must be adjusted in the field. However, for this adjustment, a high cost metering device is required.
(3) Further, if the reading out timing is changed, the reading out point of an original sheet or image is also changed.